LOCAL-FIELD EFFECTS AND CONFIGURATIONAL DISORDER IN NONLINEAR-OPTICALSYSTEMS WITH COHERENCE

We argue that in nonlinear optical systems with randomly distributed a toms one should go beyond the Clausius-Mossoti limit in order to take into account the effect of local-field fluctuations induced by configu rational disorder in atom positions. This effect is analyzed by means of a random local-mean-field approach with the neglect of the correlat ions between dipole moments of different atoms. The formalism is appli ed to three-level Lambda-type systems with quantum coherence possessin g an absorptionless index of refraction and lasing without inversion. We show that the effect of configurational fluctuations results in the significant suppression of the atom susceptibility compared to the pr edictions based on the Clausious-Mossoti equation.